1. Field of the Invention
The present invention relates to an image stabilization control circuit for driving an image stabilization mechanism provided in order to compensate for camera shake or other vibration in an image-capturing device such as a digital still camera.
2. Description of the Prior Art(s)
Contemporary image-capturing devices are often provided with camera shake correction functions in order to suppress a decline in picture quality due to camera shake. Many types of camera shake correction methods exist. In one of the methods, vibration in the image-capturing device is detected by a vibration-detection element, and an optical component such as a correction lens, or an imaging element such as a CCD image sensor is displaced by an actuator on the basis of the detected signal. The vibration-detection element employs a gyro-sensor and detects angular velocity that corresponds to the change in the direction of the optical axis. The displacement magnitude of the lens or the like is used to controllably drive the actuator. Therefore, the image stabilization control circuit for generating the driving signal of the actuator performs a process in which the angular velocity or other type of displacement velocity obtained from the vibration-detection element is integrated and converted to the displacement magnitude.
More particularly, the process for obtaining the displacement magnitude subjects the angular velocity signal outputted from the gyro-sensor to a camera shake component extraction process to remove a frequency component below the region of camera shake vibration frequencies, and converts the angular velocity into an angle-dependent displacement magnitude by integration. In the process for obtaining the displacement magnitude, by damping the output signal of the integration process or by other means, a centering process is also performed to establish the displacement magnitude so that it is made more difficult for the lens or the like to reach the movability limit. As used herein, the phrase “a processor for generating the vibration-compensating signal that corresponds to the displacement magnitude on the basis of the output signal of the gyro-sensor” is referred to as a gyro-equalizer.
Heretofore, gyro-equalizers have been implemented by software for which a microprocessor is used. In this case, a high processing rate is required for the image stabilization control circuit, and the microprocessor must be able to operate with a high speed clock. For instance, in the event that an imaging device is capturing 30 image frames per second to obtain moving images, it is necessary for the lens position to follow a vibration with a speed greater than 1/30th of a second.
Power consumption increases in the image stabilization control circuit in the event that a microprocessor is driven using a high speed clock. An image-capturing device carrying a image stabilization control circuit is driven by a secondary battery such as a lithium battery as a power source. Therefore, as the power consumption of the image stabilization control circuit is increased, the secondary battery depletes more rapidly, and the drive time of the image-capturing device is reduced. In other words, a problem arises in which the time for capturing moving pictures is reduced, and the number of capturing still images decreases. Because the camera shake correction function in an image-capturing device often operates not only when capturing moving pictures or still images but also during preview mode when an image is being prepared, consumption of power by the camera shake correction function should preferably be reduced.
In this case, by implementing a gyro-equalizer with a filter circuit, the microprocessor can be dispensed with and power consumption can be reduced. More specifically, a camera shake component extraction process can be configured using a high frequency pass filter (high pass filter, or HPF). It is possible to perform an integration process by using a low frequency pass filter (low pass filter, or LPF). It is also possible to perform a centering process by using an HPF and removing the direct-current component of the integration process output signal.
In the event that the gyro-equalizer comprises these filter circuits, it is desirable that the phase characteristic of the gyro-equalizer be 90° delayed from the input signal of the gyro-equalizer at least in the target compensation region BCMP for vibration compensation. In other words, the accuracy decreases as the phase delay (phase lag) is shifted from 90°, and vibration will be less accurately compensated.
An LPF performing an integration process, for example, has a phase characteristic such that there is a delay of 90° in a frequency range higher than a transition region in which a cutoff frequency fc is located and 0° in a frequency range lower than the transition region. A centering HPF has a phase characteristic such that there is a shift of 0° in a frequency range higher than the transition region and an advance of 90° in a frequency range lower than the transition region. Hence, in a gyro-equalizer using a filter circuit, the magnitude of the phase delay in the low frequency range falls below 90°. The decline of the magnitude of the phase delay can reach the region of the low frequency range within the target compensation region for such reasons as mentioned above; i.e., the phase characteristics of the LPF and HPF have a transition region in the vicinity of the cutoff frequency fc, and the lower limit of the target compensation region is set to a low frequency of, e.g., several Hz. FIG. 4 shows phase characteristics of a gyro-equalizer schematically illustrating such circumstances, the horizontal axis corresponding to a frequency f, and the vertical axis corresponding to a phase φ of an output signal corresponding to an input signal. In FIG. 4, a frequency fL is the lower limit of the target compensation region BCMP, and a frequency fH is the upper limit.
An angular velocity component fluctuating at a frequency lower than the lower limit fL will be removed by an HPF provided to the input of the gyro-equalizer; therefore, the effect on the accuracy of the displacement magnitude required in the gyro-equalizer caused by the fact that the phase delay is less than 90° in a frequency region below fL can be limited. By contrast, the fact that the phase characteristic in the target compensation region BCMP deviates from a 90° phase delay presents a problem in regard to a stronger effect of reducing the accuracy of the vibration-compensating signal corresponding to the displacement magnitude, and adequate vibration compensation cannot be realized.